#include <cstdio>
#include <algorithm>
#include <cstring>
#define N 10005
#define eps 1e-8
using namespace std;
struct point
{
    double x, y;
    inline double operator*(const point &rhs) const
    {
        return x * rhs.y - y * rhs.x;
    }
} po[N];
int n;
double S;
double abs(double x)
{
    return x > 0 ? x : -x;
}
double calc()
{
    double ret = 0;
    for (int i = 1; i <= n; i++)
        ret += po[i] * po[i + 1];
    return abs(ret / 2) + 0.5;
}
int main()
{
    while (scanf("%d", &n) != EOF && n)
    {
        for (int i = 1; i <= n; i++)
            scanf("%lf%lf", &po[i].x, &po[i].y);
        po[n + 1] = po[1];
        printf("%d\n", (int)calc());
    }
    return 0;
}